1. Field of the Invention
The present invention generally relates to linear image sensors and, more particularly, to a linear image sensor whose electric charge storage time can be varied.
2. Description of the Prior Art
Linear image sensors have been frequently used as image readers for a copying machine, a facsimile and so on, for example. FIG. 1 shows an example of such linear image sensor according to the prior art.
Referring to FIG. 1, it will be seen that a plurality of image sensing elements 1, 1, . . . are arrayed to form an image sensor array 2. A charge-transfer register 3 formed of a charge-coupled device (CCD) is provided on one side of the image sensor array 2 by a small distance from the image sensor array 2. A read-out gate 4 is provided between the image sensor array 2 and the charge-transfer register 3.
In this conventional linear image sensor, electric charges stored in the respective image sensing elements 1, 1, . . . are read out through the under side of the read-out gate 4 to the charge-transfer register 3 by the application of the read-out signal, which turns on the read-out gate 4, to the read-out gate 4. Thereafter, the respective electric charges are transferred by the charge-transfer register 3 and output to the outside bit by bit.
Of the video camera apparatus having a large number of image sensor arrays, such one is known to have an electronic shutter function by disposing an overflow-drain and a gate on one side of each of the image sensor arrays (see Japanese Laid-Open Patent Publication No. 62-166661).
The conventional linear image sensor shown in FIG. 1 has the disadvantage such that an output level for the same object is fluctuated depending on the light intensity of a light source for the object. That is, fluorescent lamps are frequently employed as the light source for reading the object in the facsimile and the copying machine. The fluorescent lamp, however, has the drawback such that the light intensity thereof is considerably fluctuated due to aging change. As a consequence, due to the change of the light intensity of the light source, the output level for the same object is fluctuated.
Further, since the scanning speed of the conventional linear image sensor relative to the object is not constant, the storage time of electric charges is changed with the scanning speed so that the output level is unavoidably fluctuated.